Caliste-3D CZT: development of a miniature, monolithic and hybrid gamma-ray imaging spectrometer with improved efficiency in the 100 keV to 1 MeV range and optimised for detection of the Compton effect and sub-pixel localisation

Multi-wavelength observation of astrophysical sources is the key to a global understanding of the physical processes involved. Due to instrumental constraints, the spectral band from 0.1 to 1 MeV is the one that suffers most from insufficient detection sensitivity in existing observatories. This band allows us to observe the deepest and most distant active galactic nuclei, to better understand the formation and evolution of galaxies on cosmological scales. It reveals the processes of nucleosynthesis of the heavy elements in our Universe and the origin of the cosmic rays that are omnipresent in the Universe. The intrinsic difficulty of detection in this spectral range lies in the absorption of these very energetic photons after multiple interactions in the material. This requires good detection efficiency, but also good localisation of all the interactions in order to deduce the direction and energy of the incident photon. These detection challenges are the same for other applications with a strong societal and environmental impact, such as the dismantling of nuclear facilities, air quality monitoring and radiotherapy dosimetry.

The aim of this instrumentation thesis is to develop a versatile '3D' detector that can be used in the fields of astrophysics and nuclear physics, with improved detection efficiency in the 100 keV to 1 MeV range and Compton events, as well as the possibility of locating interactions in the detector at better than pixel size.

Several groups around the world, including our own, have developed hard X-ray imaging spectrometers based on high-density pixelated semiconductors for astrophysics (CZT for NuSTAR, CdTe for Solar Orbiter and Hitomi), for synchrotron (Hexitec UK, RAL) or for industrial applications (Timepix, ADVACAM). However, their energy range remains limited to around 200 keV (except for Timepix) due to the thinness of the crystals and their intrinsic operating limitations. To extend the energy range beyond MeV, thicker crystals with good charge carrier transport properties are needed. This is currently possible with CZT, but several challenges need to be overcome.

The first challenge was the ability of manufacturers to produce thick homogeneous CZT crystals. Advances in this field over the last 20 years mean that we can now foresee detectors up to at least 10 mm thick (Redlen, Kromek).

The main remaining technical challenge is the precise estimation of the charge generated by the interaction of a photon in the semiconductor. In a pixelated detector where only the X and Y coordinates of the interaction are recorded, increasing the thickness of the crystal degrades spectral performance. Obtaining Z interaction depth information in a monolithic crystal theoretically makes it possible overcome the associated challenge. This requires the deployment of experimental methods, physical simulations, the design of readout microelectronics circuits and original data analysis methods. In addition, the ability to localise interactions in the detector to better than the size of a pixel will help to solve this challenge.

architecture for embedded system of Automated and Reliable Mapping of indoor installations

The research focuses on the 3D localization of data from measurements inside buildings, where satellite location systems, such as GPS, are not operational. Different solutions exist in the literature, they rely in particular on the use of SLAM (Simultaneous Localization And Mapping) algorithms, but the 3D reconstruction is generally carried out a posteriori. In order to be able to propose this type of approach for embedded systems, a first thesis was carried out and led to a choice of algorithms to embed and a draft of the electronic architecture. A first proof of concept was also realized. Continuing this work, the thesis will have to propose a method allowing the localization device to be easily embedded on a wide range of nuclear measuring equipment (diameter, contamination meter, portable spectrometry, etc.). The work is not limited to a simple integration phase; it requires an architectural exploration, which will be based on adequacy between algorithm and architecture. These approaches will make it possible to respect different criteria, such as weight and small size so as not to compromise ergonomics for the operators carrying out the maps and quality of the reconstruction to ensure the reliability of the input data for the Digital Twin models.

Polycrystallines perovskite layers for medical X-ray imaging: impact of doping

The CEA is a major player in research into X-ray imagers for medical applications. For several years now, our laboratory at LITEN has been working in collaboration with LETI on a new generation of direct detectors based on halogenoplumbates perovskite photoconductors for applications in radiography, mammography and cardiac imaging. The laboratory has developed several processes for manufacturing thick films (>100µm) of perovskite semiconductors with state-of-the-art performances. However, they still need to be stabilized and improved to meet the stringent specifications of medical imaging. By analogy with other semiconductors (Si, Ge, CdTe, CZT, a-Se), it seems reasonable to assume that improving the performances of perovskite detectors will require advanced control of the bulk and surface properties of the semiconductor layer.
The candidate will be inspired by the developments of the perovskite community around high-purity single crystals for gamma detectors, and will transfer this know-how to the case of polycrystalline perovskite layers for X-rays. Initially, he will study the effects of unintentional extrinsic doping linked to the environment on the performance of X-ray detectors. Secondly, it will work to reduce the unintentional intrinsic doping by developing techniques for purifying precursor materials. At the same time, particular attention will be paid to the grain boundaries of polycrystalline layers and the feasibility of passivating surface defects using chemical treatments. The layers will then be tested in X-ray or gamma-ray detector devices. A thesis launched in parallel at LETI will characterize the density and nature of intrinsic carriers as a function of material and process conditions. Depending on the progress of the thesis, the possibility of intentionally doping the perovskite material could be explored. We hope that the results obtained as part of the thesis will enable us to improve the performance of X-ray detectors in order to meet medical imaging specifications, and to develop expertise in perovskite-based gamma detection. The work will be carried out in a highly collaborative environment involving laboratories from the CEA (LITEN, LETI, IRESNE), the CNRS (Institut Néel) and foreign laboratories. The PhD student will interact with several PhD students on a common topic.

Development of a neutron/gamma coincidence measurement system for the characterization of radionuclide neutron sources

This PhD work is part of sources calibration activities at the LNHB-MA and R&D activities within the SIMRI aimed at developing neutron measurement systems for the CEA and the nuclear industry. The objective of the PhD work is to develop a new measurement system using neutron/gamma coincidences to enable the characterization of the (alpha,n)-type neutron sources. These sources consists of a homogeneous mixture of an alpha particle emitter and the target substance, the nuclei of which emit neutrons via a nuclear reaction. As for example, we can cite for example: AmBe, PuBe, CmBe, or even exotic source of high emissivity and mixing several alpha radionuclides (ex. AmPuBe). For this familly of sources, the emission of neutron by reaction (alpha,n) is in simultaneous cascade with a characteristic gamma at 4.4 MeV. The detection of the neutron and the gamma in coincidence is likely to provide information of interest in the source characterization in terms of emission rate and spectral fluence. The objective is to measure precisely gamma and neutron signatures as well as gamma/neutron intensity ratios resulting from the nuclear reaction. The new measurement device must also be able to measure neutrons emitted by the spontaneous fission reaction or by (n,2n) reaction in beryllium. Others photon emission can be also provide information of interest, ex. the emission of a gamma at 2.2 MeV resulting from the capture on hydrogen. The neutron/gamma coincidence measurements can be also used to improve the evaluation of nuclear data such as cross sections of certain elements, ex. (n,gamma) reaction on oxygen or hydrogen.

A better understanding of diffusion welding in a+ß titanium alloy

As part of a short-term nuclear project, the CEA/LITEN is supporting the manufacturing activities of a titanium alloy steam generator by HIP (Hot Isostatic Pressing). Depending on its thermal and/or thermomechanical history, the alloy Ti64 presents phases in different proportions, chemical compositions and crystallographic structures.
How does diffusion welding take place between two different phases? Is there one that cross the interface preferentially and if yes, why? Which HIP parameters have a real influence? What starting microstructure allows optimal diffusion welding?
These are the questions that the thesis should answer.

Impact of LET on biological response to Flash irradiations

Recent studies with electron and proton beams have shown that irradiation at dose rates above 40 Gy/s can be as effective in inhibiting tumor growth as irradiation at the conventional dose currently used (typically 1 Gy/min) but much less toxic to healthy tissues. This phenomenon is known as the “FLASH effect”. This effect is considered one of the most important discoveries in the recent history of radiobiology due to its potential to improve the therapeutic window between tumor control and normal tissue toxicity. Recent studies show that the biological mechanisms of the FLASH effect are linked to differential tissue oxygenation. However, the exact mechanisms of the cellular biological effects of FLASH irradiations are not completely clear and some are even contradictory.

The objective of this project is a molecular characterization of the FLASH effect on a model system perfectly controlled in vitro. FLASH irradiations of cancer cells and healthy cells will be compared to conventional dose rate irradiations using electrons and carbon ions in the two associated laboratories. The differential effect will be related to the oxygenation condition of the cells, REDOX/mitochondrial metabolism and general changes in cellular metabolism.

Study of plastic scintillators for passive and active neutron measurements

The proposed doctoral work is dedicated to optimizing non-destructive characterization methods for the quantity of plutonium in radioactive waste packages. One of the primary nuclear measurement methods to achieve this goal is based on the passive counting of coincidences between spontaneously fissioned neutrons. Most neutron measurement stations are equipped with 3He counters, which have the advantage of good detection efficiency while being less influenced by gamma radiation.
However, the price of these detectors has significantly increased in recent years, and they are relatively slow as they require prior thermalization of the neutrons to be detected. Plastic scintillators offer a 5 to 10 times less expensive alternative with equivalent detection efficiency, making them attractive for implementation in industrial stations. In exchange, they are highly sensitive to gamma radiation and the phenomenon of crosstalk (parasitic coincidences due to interactions between neighboring detectors). An innovative method for discriminating between useful and parasitic coincidences by differentiating the time of flight between neutrons and gamma radiation has been developed and validated in previous work.
There are still significant practical challenges addressed by this thesis in order to move towards an industrial neutron measurement station equipped with these scintillators, particularly for technological waste (ORANO La Hague, MELOX Cadarache). The primary objective in passive neutron measurement will be to advance in detection efficiency, data processing, and reducing uncertainties related to matrix effects and nuclear material localization, closely integrating experiments and modeling. A secondary objective of the thesis will be to demonstrate the feasibility of active neutron measurement in terms of data processing and resilience to high counting rates for quantifying the mass of nuclear material (detection of neutrons induced by a neutron generator).
The proposed doctoral work is dedicated to optimizing non-destructive characterization methods for the quantity of plutonium in radioactive waste packages. One of the primary nuclear measurement methods to achieve this goal is based on the passive counting of coincidences between spontaneously fissioned neutrons. Most neutron measurement stations are equipped with 3He counters, which have the advantage of good detection efficiency while being less influenced by gamma radiation.
However, the price of these detectors has significantly increased in recent years, and they are relatively slow as they require prior thermalization of the neutrons to be detected. Plastic scintillators offer a 5 to 10 times less expensive alternative with equivalent detection efficiency, making them attractive for implementation in industrial stations. In exchange, they are highly sensitive to gamma radiation and the phenomenon of crosstalk (parasitic coincidences due to interactions between neighboring detectors). An innovative method for discriminating between useful and parasitic coincidences by differentiating the time of flight between neutrons and gamma radiation has been developed and validated in previous work.
There are still significant practical challenges addressed by this thesis in order to move towards an industrial neutron measurement station equipped with these scintillators, particularly for technological waste (ORANO La Hague, MELOX Cadarache). The primary objective in passive neutron measurement will be to advance in detection efficiency, data processing, and reducing uncertainties related to matrix effects and nuclear material localization, closely integrating experiments and modeling. A secondary objective of the thesis will be to demonstrate the feasibility of active neutron measurement in terms of data processing and resilience to high counting rates for quantifying the mass of nuclear material (detection of neutrons induced by a neutron generator).
This work opens up career prospects particularly in research centers and R&D departments in industry.
A master internship is proposed by the team in addition to the thesis.

Imaging with Micromegas detectors with Optical readout

Recent developments have shown that coupling a Micromegas gaseous detector on a glass substrate with a transparent anode and a CCD camera enable the optical readout of Micromegas detectors with an impressive spatial resolution showing that the glass Micromegas detector is well-suited for imaging. This feasibility test has been effectuated with low-X-ray photons permitting energy resolved imaging. This test opens the way to different applications. Here we will focus, on one hand, on neutron imaging for non-destructive examination of highly gamma-ray emitting objects, such as fresh irradiated nuclear fuel or radioactive waste and on the other hand, we would like to develop a beta imager at the cell level in the field of anticancerous drug studies.
Both applications require gas simulations to optimize light yields, optimization of the camera operation mode and design of the detectors in view of the specific constraints of reactor dismantling and medical applications: spatial resolution and strong gamma suppression for neutron imaging and precise rate and energy spectrum measurements for the beta. The image acquisition will be optimized for each case and dedicated processing algorithms will be developed.

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